Size effects on the magnetic properties of a system of ε-Fe2O3 nanoparticles embedded in a SiO2 xerogel matrix

https://doi.org/10.1016/j.ceramint.2024.11.048

A representative series of samples consisting of fine ε-Fe₂O₃ nanoparticles uniformly distributed over the SiO₂ xerogel matrix with an iron oxide content of 5–33 wt % has been synthesized and studied by mutually complementary physical methods. It has been shown by the X-ray diffractometry and high-resolution electron microscopy examination that, with increasing iron oxide concentration, the average particle size <d> increases from 4 to 11 nm. According to the X-ray diffractometry and Mӧssbauer spectroscopy data, the samples with an iron oxide content of 5–20 wt % are single-phase, while at the highest Fe₂O₃ concentration (33 wt %), the β-Fe₂O₃ and α-Fe₂O₃ phases arise. As the average particle size <d> increases, a monotonic increase in coercivity H_C and remanent magnetization M_R of the synthesized systems at room temperature is observed, which is indicative of their magnetic hysteresis. The magnetic transition known to occur in the ε–Fe₂O₃ oxide manifests itself in all the investigated samples as a drastic change in the H_C and M_R values at 150–75 K. At the same time, a thorough analysis of the temperature dependence of the real part of the ac magnetic susceptibility χ′ has shown that the particles with a size smaller than a critical value of d_C ∼6.5 nm do not undergo the magnetic transition and have a much lower magnetic anisotropy constant as compared with coarser particles (d > d_C). It has been found that, in the low-temperature region, the magnetic moments of these fine particles experience superparamagnetic blocking. It has been established that the size effects that arise in ultra-fine ε–Fe₂O₃ particles has a serious impact on the macroscopic magnetic properties of the highly dispersed systems based on them.